It is a remarkable claim, but given the secrecy that has surrounded the British hydrogen weapon programme of that time - no official history existed and few documents were declassified - it has hitherto been impossible to test it.
In the past few months, however, that has begun to change and last week saw the publication of the first academic study to draw together the new evidence trickling into the public domain. In an article in the journal Contemporary Record, John Baylis, Professor of International Politics at the University of Wales, challenges the 'thermonuclear bluff' theory.
The Americans, he says, were told more about the true state of British weapons development than has previously been recognised. They would have known from an early stage that Britain was on course to make a successful H-bomb and, even if the British government had tried, it is unlikely that the Americans could have been duped.
But somebody was duped. And, on the evidence of this article, it was the British public, and, perhaps, the Russians. The Macmillan government is revealed to have been extremely economical with the truth in its statements about nuclear matters.
Britain's H-bomb programme was no simple affair. The Aldermaston scientists, it appears, suffered a serious setback in their work in the early summer of 1957, when their first hydrogen bomb tests badly failed to meet expectations.
Hydrogen bombs derive their power from the fusion, or coming together, of atoms; atomic bombs employ fission, the splitting of atoms. H-bombs are vastly more destructive, with explosive yields up to 1,000 times that of the bombs which were dropped on Hiroshima and Nagasaki.
In July 1954 the British government decided it must follow the US and the Soviet Union in developing these weapons. The attraction, as the Chiefs of Staff explained, was that hydrogen weapons did not suffer the 'problem of terminal accuracy' that afflicted atomic bombs - that is, they were so destructive you could miss your target by a long way and still be sure of obliterating it.
In great haste and secrecy, the scientists of the Atomic Weapons Research Establishment set about mastering the necessary science and technology, racing to produce a weapon for testing before the mounting pressure of international public opinion brought about a moratorium on atmospheric nuclear tests.
Professor Baylis shows that by 1957 they had produced a variety of designs, both for 'two-stage' weapons in which the fusion was set in motion by an initial fission, or atomic, explosion, and for 'three-stage' weapons which increased the yield with a further fission phase.
The scientists were helped, he says, partly by information leaked from the infamous Oppenheimer security hearings in the US, and partly by radiochemical data gathered from the airborne debris of other countries' bomb tests.
In May and June 1957, off Malden Island in the Pacific Ocean, the first British tests were conducted. The great haste had led the scientists to make an ambitious leap direct to detonating a device dropped from a plane at high altitude, rather than starting with a simpler explosion on the ground. There were three explosions, and they left many British scientists deeply disappointed. The yields, declassified for the first time by the Government only a few months ago, were well below expected levels.
The first and third were two-stage weapons and it seems that yields of about one megaton were expected. In the event, the figures were 0.3 megaton and 0.2 megaton. The second test, of a new design of pure fission weapon, produced the much higher yield of 0.72 megaton.
Despite this, the government in London boldly declared the thermonuclear tests a success, although the definition here is a fine one. Bombs employing fusion had been successfully dropped from planes and had successfully exploded; the fact that they had failed to produce the yield that is normally associated with hydrogen weapons was obscured.
While work proceeded on correcting the H-bomb design, Britain now developed what was called an 'interim' weapon intended to provide the RAF with a high-yield weapon at the earliest date. This interim weapon followed the pure fission design used in the second Malden Island test - it was a very big atom bomb, not a hydrogen bomb at all.
This, says Professor Baylis, is what has led to the theory of the British hydrogen bomb bluff. Saying it had successfully tested hydrogen weapons and was introducing a service weapon with a yield 'in the megaton range', the theory runs, was a piece of sleight-of-hand by Macmillan, intended to convince the Americans that the time had come to pool their knowledge with the British.
But the bluff argument, he writes, 'ignores the scale of co-operation achieved by this time' between British and American scientists. It is true that a formal Anglo-US nuclear weapons co-operation agreement was not signed until 1958, but in 1957 they were already pooling intelligence information and other data. More to the point, US experts had been present at the Pacific tests, had made their own estimates of the yields, and were 'impressed by the progress made by the British'.
This progress was soon evident. In November 1957, five months after the Malden Island tests, British scientists had made substantial advances and were able to stage a 1.8 megaton test, this time off Christmas Island. This leap forward, it seems, may have owed something to the acquisition of an IBM 704 computer from the US. The following April there was a three megaton test and by the time, soon after that, that atmospheric testing came to an end, Britain had acquired the basis for its own hydrogen bomb.
Professor Baylis also challenges claims, based on documents declassified in the US, that after passing this milestone in 1958 and signing the co-operation deal with the Americans, Britain ditched its own H-bomb design in favour of a US one. A US design may have been used in the first British H-bomb, but this 'appears to have been replaced later by an all-British one'.
And he defends Macmillan's public misrepresentation of British capabilities. 'If deterrence was a psychological game, Britain had its part to play in emphasising, perhaps even exaggerating, the disastrous consequences of aggression,' he concludes. Even if you don't have a big stick it is sometimes wise to say you do.
Brian Cathcart's book, 'Test of Greatness: Britain's Struggle for the Atom Bomb', is published on Thursday by John Murray, price pounds 19.99.
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